1. Field of the Invention
This invention relates generally to ferrite phase shifters and their method of fabrication and more particularly to a method of fabricating ferrite toroids for phase shifters using low temperature cofired ceramic (LTCC) tape technology.
2. Description of the Prior Art
Ferrite phase shifters are well known devices in which the phase of an electromagnetic wave of a given frequency propagating through a transmission line can be altered. Such devices have been extensively used in radar applications for electronic beam steering and phased array application. Two types of electronic phase shifters are currently utilized for modern phased array antenna systems, namely, ferrite phase shifters and solid state semiconductor phase shifters. Ferrite phase shifters generally fall into two categories: toroid phase shifters enclosed within a waveguide structure, and phase shifters built using transmission line microstrip configurations.
The most costly item in the fabrication of toroid ferrite phase shifters is the fabrication of ferromagnetic toroids which typically involves powder processing, sintering and expensive machining using precision diamond cutting and grinding tools.
A typical fabrication technique for producing a ferrite toroid is as follows. Oxide raw materials are first ground and wet-mixed in a ball mill lined with rubber or plastic to prevent contamination. An appropriate amount of organic binder system, solvent and water are next added during the mixing process of the powder to form a slurry which is then subjected to spray drying. The solvents are then evaporated from the atomized liquid droplets and spherical powders of the mixture of oxides and binders are formed in a spray-drying chamber. Finally, the powders are collected, poured into dyes, pressed into shape and sintered.
Another approach is to mix the ceramic, binder, plasticizer and solvent in such a way that the mixture becomes a plastic which can then be extruded in the form of bars and rods of various dimensions, after which they are dried and fired.
The firing temperature of ferrite varies between 1100.degree. C. and 1500.degree. C. and requires a precise firing schedule and oxygen partial pressure to carefully control the chemical valance state of the ferrite compounds. During firing, the oxides react with each other to form what is commonly known as ferrite.
Redimensioning and reshaping of fired ferrite devices to make square holes through the length of the structure and to maintain close tolerances are the most important and most expensive steps in the fabrication of ferrite toroids and are typically accomplished by cutting and grinding using diamond blades. Tooling and labor costs in this latter phase of fabrication are relatively high.